The distributed control system is used for plant operation control in a wide variety of fields such as petrochemistry, steel, paper pulp, foods, pharmaceuticals, and electric power.
FIG. 1 shows an exemplary configuration of a general distributed control system.
Referring to FIG. 1, an operation monitoring apparatus 1 and a controller 2 are connected to a control bus 3. The controller 2 controls a plant 4 under the monitoring of the operation monitoring apparatus 1. The operation monitoring apparatus 1 is in charge of operation and monitoring of the plant. The operation monitoring apparatus 1 displays a screen to perform control operation and monitoring. In accordance with the scale of the plant, a plurality of controllers are distributed across the plant. The operation monitoring apparatus 1 and the controller 2 communicate with each other to control the plant via the control bus 3.
Sensor devices 5, 6 present in the plant 4 detects the process values of temperature, pressure and liquid level. Valves 7, 8 have their throttle opening controlled by an operation signal given by the controller 2. An analog signal of 4 to 20 mA and 1 to 5V output from the sensor devices 5, 6 is input to the controller 2. Based on this input, a control unit (not shown) in the controller 2 performs control arithmetic operation and obtains an operation amount. The operation amount is output as an analog signal of 4 to 20 mA and 1 to 5V, which controls the throttle opening of the valves 7, 8. For example, by controlling the valve throttle opening of a reaction oven the process amount of temperature or pressure is controlled.
The control bus of a conventional distributed control system is a bus dedicated to process control. The protocol for the control bus is a protocol dedicated to process control. In recent years, remarkable progress in IT (Information Technology) or web-related technologies has presented a need for the open architecture of the control bus for a distributed control system. As a request for the open architecture, a request for a network that is based on Ethernet (registered trademark) is on the rise.
With this background, it is under study to apply Industrial Ethernet(registered trademark) to the control bus for a distributed control system.
The Patent Reference 1 describes a communication control system that employs a redundant configuration of a network adapter transparent to a user application program when a communication station communicating over the TCP/IP (Transmission Control Protocol/Internet Protocol) is connected to Ethernet(registered trademark).
Patent Reference 1: JP-A-11-205356
Industrial Ethernet(registered trademark) does not currently satisfy the real time property or reliability requested in process control so that it is difficult to provide such a communication control system.
In a conventional system, it is difficult to implement open protocol communications while assuring real time property and reliability from the following circumstances:    (a) Communications over a standard protocol that is based on Ethernet(registered trademark) or TCP/IP (Transmission Control Protocol/Internet Protocol) do not sufficiently satisfy the needs for real time property and reliability when used in industrial applications such a process control.    (b) Communications over a unique dedicated protocol for attaining real time property and reliability have only insufficient compatibility when connected to another system or when a network device or software widely available on the market is used. This does not sufficiently meet the request for open protocol communications.
The invention has been accomplished in order to solve the problems. An object of the invention is to provide a communication control system capable of performing communications that satisfy real time property, high reliability and a need for open protocol communications by introducing in the same communication station critical communication section for providing real time property and high reliability as open communication section.